How can I scale Kafka consumers?
TL;DR
Topic is made up of partitions. Partitions decide the max number of consumers you can have in a group.
Scenario 1:
When we have only one consumer, It can read all the messages from all the partitions.
Scenario 2:
In the above set up, when you increase the number of consumers in the group, partition reassignment happens and instead of consumer 1 reading all the messages from all the partitions, consumer 2 could share some of the load with consumer 1 as shown below.
Scenario 3:
What happens If I have more number of consumers than the number of partitions.? Each consumer would be assigned 1 partition. Any additional consumers in the group will be sitting idle unless you increase the number of partitions for a Topic.
Summary:
We need to choose the partitions accordingly. That decides the max number of consumers in the group. Changing the partition for an existing topic is really NOT recommended as It could cause issues.
That is, Let's assume a producer producing names into a topic where we have 3 partitions. All the names starting with A-I go to Partition 1, J-R in partition 2 and S-Z in partition 3. Let's also assume that we have already produced 1 million messages. Now if you suddenly increase the number of partitions to 5 from 3, It will create a different A-Z range now. That is, A-F in Partition 1, G-K in partition 2, L-Q in partition 3, R-U in partition 4 and V-Z in partition 5. Do you get it? It kind of affects the order of the messages we had before! So you need to be aware of this. If this could be a problem, then we need to choose the partition accordingly upfront.
More info is here - http://www.vinsguru.com/kafka-scaling-consumers-out-for-a-consumer-group/
Your assumption about messages being consumed twice is correct (since each group consumes 100% of messages from a topic).
I agree with David. Moreover, I suggest that you create more partitions than you really need, which would leave you some headroom to increase the number of threads in the group when such a need arises.
You can always increase the number of partitions later (and/or add additional brokers), but it's nice to have that already done, so that you can only increase number of threads and be done with it (those situations usually require a quick response, so you should do all the prep. that you can do in advance).